Integrated optical circuits provide signal processing for fiber optic systems such as fiber optic gyroscopes. Such integrated optical circuits comprise a substrate that carries light propagating and light processing elements. Typically, a waveguide is embedded in a surface region of the substrate for propagating light, and processing elements such as light splitters, phase modulators, or delay lines, are disposed along the waveguide to provide the desired signal processing. Input and output optical fibers are optically coupled to the substrate to provide an input optical signal, and to receive a processed output signal, respectively. While it is intended that light be within the substrate in specific regions (e.g., waveguide and associated processing regions), as a practical matter light propagates elsewhere in the substrate, as well as in the desired regions.
Light that travels along paths other than the waveguide, or desired processing regions, i.e., "stray light", can degrade the quality of the optical signal and affect processing. The major source of stray light is imperfect optical coupling between the optical fibers and the waveguide, although stray light can also be introduced by other mechanisms such as by spurious radiation from the signal processing elements. Stray light will corrupt the optical signal if the stray light is transferred to the waveguide or to the output optical fiber.
One method of reducing the effect of stray light on the optical signal is to roughen, or "texture", outer surfaces of the substrate and to coat those outer surfaces with a light absorbing material. This method relies on absorbing the stray light rays that strike the outer surfaces of the substrate. This method is generally ineffective because, as is described by Fresnel's Reflection Law, a light striking a surface has an absorbed component and a reflected component. The strength of the reflected component is dependant upon the angle of incidence, such that the strength of the reflected component decreases as the angle of incidence approaches 90 degrees. Since, in typical situations, much of the stray light is incident on the outer surfaces of a substrate at oblique angles, strong reflected components are generated and are propagated back into the substrate, despite the surface coating of light absorbent material. The ineffectiveness of this method is further compounded by mismatches between the optical refractive indexes of the substrate and the absorber. Typically, no more than 20% of the stray light can be absorbed if conventional absorbers are used, and the stray light has an angle of incidence of less than 20 degrees.
Another method of reducing the deleterious effects of stray light is to adjust the alignment between the input optical fiber and an optical waveguide in the substrate so as to minimize the stray light produced at the coupling. Unfortunately, minimizing the stray light in this way also reduces the efficiency of the coupling between the optical fiber and the waveguide, so stray light is reduced at the expense of lowering the signal strength. Also, computer simulations indicate that the reduction of stray light achieved by this method is marginal at best.
Thus there is a need for a better method of reducing the deleterious effects of stray light in optical substrates.